This invention relates to memory modules, and more particularly for stacking memory chips on memory modules.
Memory modules are extremely popularity today. Most personal computers (PC""s) are shipped with sockets for memory modules so the PC user can later add additional modules, increasing the memory capacity of the PC. Other devices may also use memory modules designed for PC""s. High-volume production and competition have driven module costs down dramatically, benefiting the buyer.
Memory modules are made in many different sizes and capacities, with the older 30-pin modules being replaced by 72-pin and 168-pin modules. The pins were originally pins extending from the module""s edge, but now most modules are leadless, having metal contact pads or leads. The modules are small in size, some being about 5.25 inches long and 1.2 or 1.7-inch high.
The modules contain a small printed-circuit board substrate, typically a multi-layer board with alternating laminated layers of fiberglass insulation and foil or metal interconnect layers. Surface mounted components are soldered onto one or both surfaces of the substrate. Dynamic-RAM integrated circuits (IC""s) or chips are commonly packaged in inexpensive surface-mount packages such as small-outline J-leaded (SOJ) packages, Plastic Leaded chip carriers (PLCC""s), Thin Small-Outline Packages (TSOP) or small-outline (SO) packages.
The number of DRAM chips in a module depends on the capacity and data-width of the DRAM chips and the size of the memory module. For example, a memory module constructed from 64 Mega-bitxc3x974-bit-output DRAM chips needs 16 of these 4-bit-wide DRAM chips to fill a 64-bit bus. The module has a capacity of 512 Megabytes (MB).
A memory module can have more than one bank. A two-bank memory module with a total capacity of 1024 MB can have two banks of 16 DRAM chips per bank, using 64Mxc3x974 chips.
FIG. 1 is a schematic of a memory module with two banks of DRAM chips. DRAM chips 12-16 form a first bank, while DRAM chips 22-26 from a second bank. A total of 16 DRAM chips 12-16 are in the first bank, while another 16 DRAM chips 22-26 form the second bank. When 64Mxc3x974 DRAM chips are used, each bank contains 512 MB.
Most signals are shared by all DRAM chips in both banks. For example, control signals such as RAS (row address strobe), CAS (column address strobe), write-enable, clocks, etc. and address lines are connected to all DRAM chips in both banks on the memory module. Data lines are each shared by one chip in each of the banks. Data lines D0-D3 are shared by DRAM chip 12 in the first bank and DRAM chip 22 in the second bank. Likewise, data lines D4-D7 are shared by DRAM chips 13, 23.
The banks are selected by chip-select signals. Only one chip select is activated at a time for the memory module. Chip select CS1 activates DRAM chips 12-16 in the first bank, while chip select CS0 selects DRAM chips 22-26 in the second bank. When the bank""s chip select is not activated, the control signals are ignored by DRAM chips in the bank. The data lines are not driven by the non-selected bank of DRAM chips to prevent data conflicts.
FIG. 2 shows a prior-art double-bank memory module with stacked DRAM chips. While a 2-bank memory module can be constructed from non-stacked chips, twice as much surface area on the substrate would be needed. The memory module contains a substrate 10, with surface-mounted DRAM chips 22-26 mounted directly to the front surface or side of substrate 10, while more DRAM chips (not visible) are mounted to the back side or surface of substrate 10. Eight stacks of DRAM chips can be mounted on the front surface of substrate 10 for bits 0-31, while another eight stacks of chips can be mounted on the back side for bits 32-63. Metal contact pads 20 are positioned along the connector edge of the module on both front and back surfaces. Metal contact pads 20 mate with pads on a module socket to electrically connect the module to a PC""s motherboard. Holes and/or notches are sometimes used to ensure that the module is correctly positioned in the socket.
Often the number of DRAM chips desired on a memory module is larger than the available substrate surface area. One method to squeeze more DRAM chips to a module is to stack DRAM chips on top of one another. For example, DRAM chip 12 can be mounted directly on top of DRAM chip 22. The leads or pins of upper DRAM chip 12 can be soldered to the leads or pins of lower DRAM chip 22.
Usually DRAM chips from separate banks are stacked together. The DRAM chips stacked together share the same data lines as well as control signals. DRAM chips 12-16 of the first bank are stacked on top of DRAM chips 22-26 of the second bank. When chips are mounted to both surfaces of substrate 10, two chips can be stacked together on the front surface and two chips can be stacked together on the back surface at each location of the substrate. Each surface can have mounting locations for 8 DRAM chips, for a total of 16 mounting locations (only 5 locations are shown in the FIG.). With stacking, a total of 32 DRAM chips can be mounted to substrate 10.
FIG. 3 is an exploded view showing stacking of a pair of DRAM chips. Upper DRAM chip 12 of the first bank is mounted on the top of the stack, with lower DRAM chip 22 mounted below to the module substrate. One side of pins is shown, but pins are usually on two or even on all four of the four sides of a DRAM IC package.
Between upper DRAM chip 12 and lower DRAM chip 22 is thin printed-circuit board (PCB) 30. Thin PCB 30 is a thin circuit board that can be made from fiberglass with printed metal layers on its two major surfaces. Thin PCB 30 has bonding pads 40 on its upper surface that are arranged for making contact with the pins S1-S7 from upper DRAM chip 12. The pins from upper DRAM chip 12 are soldered to these bonding pads 40 on the upper surface of thin PCB 30 during manufacturing.
The bottom surface of thin PCB 30 also has leads C1-C7 that are arranged to make contact with the tops of the pins S1-S7 of lower DRAM chip 22. These leads C1-C7 of thin PCB 30 are soldered to the pins S1-S7 of lower DRAM chip 22. Thus thin PCB 30 has bonding pads on the top surface that are soldered to pins of upper DRAM chip 12, and has leads that are soldered to pins of lower DRAM chip 22.
Each of the leads is connected to an upper bonding pad either directly above or through a drilled via or a metalized connection through the substrate of thin PCB 30. Thus thin PCB 30 electrically connects pins from lower DRAM chip 22 to the pins of upper DRAM chip 12 that are directly above.
All of the address, data, RAS, CAS, clock, power, ground, and most other signals are shared among the two stacked DRAM chips 12, 22 in this way by directly connecting corresponding pins S1-S7 in upper and lower DRAM chips 12, 22. For example, pin 1 (S1) of upper DRAM chip 12 caries signal SI and connects through the first of bonding pads 40 on the top surface of thin PCB 30 to lead C1 which is soldered to pin 1 (signal S1) of lower DRAM chip 22.
While most pins of lower DRAM chip 22 are connected with the pins directly above of upper DRAM chip 12, there are some exceptions. The chip-select pins are disconnected and re-routed by thin PCB 30 so that the stacked DRAM chips receive different chip-select signals. This allows one of the DRAM chips 12, 22 to be selected and the other de-selected.
For example, chip select CS0 connects to lower DRAM chip 22 through pin 3. Chip select CS0 controls the second DRAM bank that includes lower DRAM chip 22. The connection from CS0 to thin PCB 30 is broken by the removal of lead C3 of thin PCB 30. Without the C3 lead, no connection is made between pin 3 of lower DRAM chip 22 and thin PCB 30.
DRAM chips often include unused pins known as no-connect (NC) pins. These pins do not connect to circuitry inside the DRAM chip, but are still present as physical pins or leads on the package. In this example, DRAM chips 12, 22 have pin 5 as a NC pin.
The chip-select CS1 signal from the memory module substrate is connected to pin 5 of lower DRAM chip 22. Rather than leave an isolated bonding pad on the substrate of the memory module under pin 5, this bonding pad is connected by metal traces on the memory module substrate to the CS1 signal line. Since pin 5 is a NC pin, lower DRAM chip 22 ignores this CS1 signal.
However, pin 5 of lower DRAM chip 22 is soldered to lead C5 of thin PCB 30. Wiring trace 32 on thin PCB 30 electrically connects lead C5 to another one of bonding pad 40, the bonding pad with the missing C3 lead. This bonding pad is soldered to pin 3 of upper DRAM chip 12. Thus the CS1 signal from the memory-module substrate is routed up through the no-connect pin 5 of lower DRAM chip 22 to lead C5 of thin PCB 30. Then wiring trace 32 moves this CS1 signal from C5 to the bonding pad for the missing C3 lead. From this bonding pad for C3, the CS1 signal is carried to pin 3 of upper DRAM chip 12. Pin 3 of DRAM chips 12, 22 is the chip-select input.
Thin PCB 30 allows lower DRAM chip 22 to receive CS0 at it pin-3 chip-select input, while routing CS1 from an unused pin (5) of lower DRAM chip 22 to the pin-3 chip-select input of upper DRAM chip 12.
FIGS. 4A-B show cross-section views of stacked DRAM chips with a thin PCB in-between. FIG. 4A shows separated components while FIG. 4B shows the final assembled stack. Lower DRAM chip 22 has pins 46 that are soldered to metal pads on substrate 10 of the memory module. Upper DRAM chip 12 has pins 42 that are for soldering to bonding pads 40 on the top surface of thin PCB 30xe2x80x2. Bonding pads 40 may be the upper part of leads 44. Leads 44 can be J-leads that have bottom ends that are for soldering to the top shoulders of pins 46 of lower DRAM chip 22.
Soldering leads 44 to the upper shoulders of pins 46 can be difficult due to the small size of pins 46 and the small shoulder region of these pins 46. FIG. 5 is another prior-art embodiment of stacked DRAM chips using L-leads. Rather than use J-leads that attach to pin shoulders, thin PCB 30xe2x80x2 has L-leads 48 that are soldered to metal pads 45 on the top of substrate 10 of the memory module. Pads 45 can be enlarged to make room for the foot of L-leads 48 next to pins 46 from lower DRAM chip 22.
Via 41 can be drilled through thin PCB 30xe2x80x2 and metalized to make contact from bonding pads 40 on the upper surface of thin PCB 30xe2x80x2 to L-leads 48 that are on the bottom of thin PCB 30xe2x80x2. A conductive material may be incorporated into thin PCB 30xe2x80x2 during manufacture of thin PCB 30xe2x80x2 rather than drilling vias 41.
Other prior-art methods to stack DRAM chips include using a lead-frame rather than thin PCB 30. The lead-frame can have leads that are soldered to the shoulder of the pins of lower DRAM chip 22. The pins of upper and lower DRAM chips 12, 22 can also be directly soldered together without an intervening thin PCB by bending pins of the upper DRAM chip. To re-route the chip select signals, pins can be bent or cut and a wire jumper used, or a much smaller PCB insert is inserted between the pins and the DRAM package, perpendicular to the direction of thin PCB 30.
While these stacking techniques are useful, low yields and complexity can hinder their economy. What is desired is an improved DRAM-stacking technique.